The improper handling and disposal of hazardous wastes has resulted in thousands of contaminated sites in the United States and throughout the world. Groundwater and soils contaminated with organic compounds, such as solvents, are often difficult to remediate. Methods for the cleanup of these sites have evolved over the past 25 years; the first approaches focused on excavation of source areas and the pump-and-treat cleanup of groundwater. The depth and areal distribution of contaminants often precludes any attempts at excavation while the effectiveness of groundwater extraction is limited by the low solubility of these contaminants, the weakness of dispersive mixing processes, and kinetic limitations on the rates of mass transfer from the non-aqueous phase into the dissolved phase. Recently, in-situ chemical oxidation (ISCO) using oxidizers such as permanganate, hydrogen peroxide, or ozone have been deployed to aggressively remove contaminant mass and reduce the concentration of the target compounds below regulatory criteria.
In-situ chemical oxidation involves the introduction of a chemical oxidant into the subsurface to transform contaminants into species that are harmless or non-objectionable by treating them in-situ, or in place. Chemical oxidation is a process in which the oxidation state of a substance is increased and the oxidant is reduced by accepting electrons released from the transformation (oxidation) of target and non-target reactive species. For example, oxidation of trichloroethylene (TCE) and perchloroethylene (PCE) with ozone may produce reaction byproducts that include dichloroacetaldehyde and dichloroacetic acid, compounds with lower toxicity. Oxidation of these byproducts to CO2 and H2O could also be accomplished through continued reaction with ozone.
Injecting ozone gas into saturated soils, also known as ozone sparging, has been applied at many sites with various level of success. In conventional ozone sparging, ozone is produced on site and injected into one or more ozone injection or sparge wells. The injection wells are constructed of solid casing to a depth that corresponds to the soil and/or groundwater contamination. A well screen or a diffuser is installed at the bottom of the injection well to allow the ozone gas to move into the subsurface. The injected gas moves into the saturated soils and rise to the top of the groundwater. Contaminants that contact the ozone gas are either oxidized in the soluble phase or volatized and transferred to the gas phase where the chemical reacts with the ozone.
Because ozone is injected into the subsurface as a gas, mass transfer of the ozone from the gas phase into the water phase is required. The most common means of accomplishing ozone introduction are sparging into the aquifer, using designs similar to those successfully used with in situ air sparging, or injection of ozonated water into the aquifer. In-situ sparging is more commonly used because of the high oxidizer demands associated with most aquifer systems and the short half-life of ozone in water.
Ozone sparge points may be either vertical or horizontal wells that are screened within the saturated zone. Often numerous wells are operated as “clusters” using a cycling schedule. Depending on the soil permeability, ozone sparge rates can range from ¼-2 scfm for most applications.
The most common type of ozone generator used in ozone sparging applications is the corona discharge generator. This generator produces ozone by passing dry air or oxygen between two concentric metallic electrodes of opposite charge, resulting in the production of ozone. Air fed systems produce 1 to 2% (by weight) ozone gas. By using concentrated oxygen as the carrier gas for ozone production, ozone concentrations as high as 14% can be achieved. Ozone generator capacities are typically expressed in terms of mass output such as pounds of ozone per day (lb/day).
The area that the rising ozone gas treats is referred to as the radius of influence (ROI). This area is often described as being symmetrical and circular, but it is rarely uniform due to subsurface heterogeneities. For this reason, the term zone of influence (ZOI) is used because it more accurately describes the processes occurring in the subsurface. The ZOI for ozone sparging is a dynamic parameter because of the highly reactive nature of ozone. As ozone moves through the subsurface, it reacts with organic matter, calcium carbonate, metals, and other constituents, thereby reducing the concentration. As more ozone is injected into the subsurface, the compounds that react with the ozone are depleted, thereby allowing the ozone to travel farther from the injection point. Eventually, the ZOI for ozone sparging may approach the ZOI of a conventional air sparge well. However, due to the highly reactive nature of ozone and its limited half-life in water (approximately 30 minutes in clean water), it is typical to encounter a ZOI for ozone that is ⅓ to ⅔ that for a conventional air sparge well.
The distance that a sparged gas moves from the injection well is a function of the depth below the water table the gas is injected, the injection pressure and the gas flow rate. Increasing the depth of gas injection is often used to increase the ZOI. However, this method of increasing the ZOI is often limited by geologic conditions (e.g. impermeable zones) and the fact that as ozone moves through saturated soils, it is depleted as it reacts with target and non-target compounds.
One of the most common limitations with ozone injection systems is the operating flow rate of the ozone generator. A typical 1 lb/day ozone generator using oxygen as the carrier gas operates at a flow rate of 5 to 20 liters per minute (LPM) which is too low to produce a ZOI greater than 3 to 5 feet. In order to overcome this limitation, compressed air is often added to the ozone gas to increase the injection flow rate, thereby expanding the ZOI. This practice also has the undesirable effect of reducing the ozone gas concentration which reduces the mass transfer efficiency. For a typical site, the reduction in ozone gas concentration is substantial—from 50,000 parts per million (ppm) to less than 1,000 ppm.